Simple textbook models of radio receivers typically use homodyne detection. Homodyne detection involves directly demodulating a radio frequency (RF) signal to baseband in a single operation. A receiver that implements homodyne detection is commonly referred to as a direct conversion receiver (DCR). Current state of the art radio receivers for Global System for Mobile Communication/Enhanced Data Rates for GSM Evolution (GSM/EDGE) use DCR systems to obtain cost reductions compared to heterodyne systems.
An emerging trend in the industry is the move toward an all-digital interface between the radio frequency (RF) section of a transceiver and the baseband processing section of the transceiver. For the receiver in particular, there is also a trend toward implementing the signal filtering in the digital domain with a minimal amount of analog processing prior to the analog-to-digital converter (ADC). This requires the ADC to provide wide dynamic range to pass out-of-band signals. To this end, it is desirable to minimize the interaction between the RF section and the baseband processing section of the receiver. Ideally, the baseband processing section would communicate only time and channel information to the RF section, and the RF section would deliver demodulated and decoded digital data to the baseband processing section.
In current transceivers, one of the reasons for non-digital interaction between the RF section and the baseband processing section is to generate what is referred to as an automatic gain control signal for certain RF receiver components. Typically, a received signal is analyzed to determine the amount of energy in the signal. An automatic gain control (AGC) circuit is generally driven by an estimate of the energy in the received signal. This typically requires that the receiver make a determination of the amount of energy in the desired signal and adjust the gain of the adjustable receiver components so that the mean signal strength remains at an approximate target value. The baseband processing section typically generates what is referred to as a received signal strength indicator (RSSI) signal. Information relating to the strength of the received signal is then communicated back to the RF section so that adjustments to components in the receiver can be made.
Unfortunately, this approach has deficiencies. The energy in the desired signal must be estimated. As the desired signal at the antenna input also contains signals at other frequencies, it is necessary to first remove these “out-of-band” signals using costly and inefficient filters. This filtering also adds a processing delay. Then, to determine the energy in the signal, a sum of squares calculation is typically performed on the desired signal, consuming processing resources.
In addition, the energy estimate is often filtered by a lowpass filter to determine the mean level of the desired signal over a time interval, thus introducing an additional delay. Further, a separate controller is often used to perform the energy filtering and to subsequently determine the appropriate receiver gain setting. Transferring data to and from the controller adds yet additional delay. The impact of the combined delay prevents the AGC circuitry from quickly responding to changes in the level of the received signal.
Finally, because the energy estimate is calculated after the received signal has been filtered, the AGC circuitry cannot compensate for changes in the level of out-of-band signals that accompany the desired signal. This necessitates that the receiver be designed with sufficient dynamic range to accommodate a wide range of out-of-band energy levels or to have sufficient filtering to remove out-of-band signals early in the receiver chain.
Thus, it would be desirable to minimize the non-digital interaction between an RF section and a baseband processing section of a transceiver.